The present invention relates to screen printers and screen printing methods for printing paste such as solder paste and conductive paste onto a board.
Screen printing is one method of printing paste such as solder paste or conductive paste onto a board during the electronic component mounting process. A mask plate provided with pattern holes on a printing area is placed on the board, and paste is printed onto the board through the pattern holes on the mask plate using a squeegee.
One known screen printing method is to use a sealed squeegee head. In ordinary screen printing, paste is directly provided on the mask plate. However, in screen printing using the sealed squeegee head, paste is stored in the squeegee head. Paste inside the squeegee head is pressurized, while an opening on the bottom face of the squeegee head provided as a paste contact face, directly contacts the mask plate. This allows paste to be dispensed through the opening on the squeegee head and into the pattern holes on the mask plate. Each pattern hole is filled with paste as the squeegee head slides across the mask plate.
However, the conventional screen printing using the sealed squeegee head has following disadvantages.
FIGS. 6A and 6B show a partial sectional view of the squeegee head in a conventional screen printer. In both Figures, sealed squeegee head 510 has tank 520 for storing solder paste 580. Pressurizing means extrudes solder paste 580 to paste container 530. Paste container 530 allows solder paste 580 to contact the surface of mask plate 550 through opening 540 created on its bottom face so that solder paste 580 is dispensed into the pattern holes.
The width of opening 540 is set to match the maximum printing width of board 560 to be printed. As shown in FIG. 6B, if the width of board 560 to be printed is narrow, only a portion of solder paste 580 in container 530 within the printing width is dispensed into the pattern holes on mask plate 550. Accordingly, if printing of board 560 with a narrow width continues, the remaining portion of solder paste 580 (the double hatched portion in FIG. 6B) supplied to paste container 530, which exceeds the board width, remains in the paste container 530 for a long period without being consumed in the printing process.
This remaining portion of solder paste 580 hardens and deteriorates as time passes, becoming unusable, requiring eventual disposal. A screen printer using a conventional sealed squeegee head thus retains a portion of solder paste due to the structure of the squeegee head and thus wastes solder.
The present invention solves the above disadvantage, and aims to offer a screen printer and a screen printing method that prevents the retention of deteriorating solder paste that eventually requires disposal.
The screen printer of the present invention prints paste on a board through pattern holes on a mask plate by sliding a squeegee head on the mask plate. The squeegee head includes:
(a) a paste feeder for supplying pressurized paste;
(b) a paste container for storing pressurized paste and making the paste contact the surface of the mask plate through an opening of a predetermined width created on its bottom face; and
(c) a paste guide for leading the paste in the paste feeder to the paste container from both outer parts in the width direction inward towards the center.
The screen printing method of the present invention prints paste on the board through the pattern holes on the mask plate by sliding the squeegee head on the mask plate, and includes the steps of:
(a) supplying pressurized paste using a paste feeder in the squeegee head;
(b) storing pressurized paste in the paste container in the squeegee head, and making the paste contact the surface of the mask plate through an opening of a predetermined width created on the bottom face of the paste container;
(c) leading paste from the paste feeder to the paste container from both outer parts in the width direction inward towards the center; and
(d) printing paste on the board in accordance with a time-series feeding sequence in Step (c).
The present invention thus prevents the degradation and the consequent need for disposal of paste due to its retention in the squeegee head for long periods.